This invention relates to a security system, of the type that monitors a series of transponders located in an area or premise to be secured, having enhanced protection against unauthorized tampering and compromising.
Security systems, which constitute data communication systems, have been developed wherein a controller monitors, and receives data over a pair of line conductors from, remote parallel-connected transponders each of which is located within the same protected premise. The term "transponder" signifies a unit which can control and/or monitor some condition and/or associated component, such as a transducer, which may or may not be adjacent to its physical location and which may or may not be within its physical enclosure. A transponder may be selectively addressed by the controller and recognizes not only its address but other information which may be transmitted from the controller, such as command signals for controlling the operation of the transponder itself and/or various associated devices, such as relays, visual and/or audible indicators, or any other device. In addition, the transponder itself may transmit information, such as the transducer response or status or any other data, back to the controller.
A transducer, associated with a transponder, may take any one of a wide variety of different forms. For example, a transducer may be an intrusion detector such as an ultrasonic space detector or an infrared space detector that detects movement within a given area, or an unauthorized entry sensor such as a reed switch actuated by a magnet (usually used on windows and doors), window tape in the form of metal foil which breaks if a window is broken, or a wire running through a screen which is cut when the screen is ripped. A transducer could also be a physical switch, such as a "holdup button" in a bank which may be manually actuated by a bank employee if a robbery occurs. The transducer could also constitute a transistor switch that is operated by some device to detect some alarm condition or state. Moreover, fire and smoke detection may also be included in the security system for the protected premise, in which case a transducer would take the form of a fire or smoke detector.
Data communication systems, which may function as security systems, are disclosed in U.S. Pat. Nos. 4,394,655, 4,470,047 and 4,507,652, and in U.S. patent application Ser. No. 716,799, filed Mar. 27, 1985, which issued Apr. 14, 1987, as U.S. Pat. No. 4,658,249, and Ser. No. 832,624, filed Feb. 25, 1986, which are assigned to the present assignee. In these patents and applications, the teachings of which are incorporated herein by reference, a controller communicates with a series of individually addressable transponders, located within the same protected premise, by sending successive composite signals, each comprising a group of pulses, over a two-wire distribution system. High-amplitude portions of a composite signal or pulse group are employed to transmit commands from the controller, while low-amplitude portions are used to return information from the addressed transponder to the controller.
Preferably, a security system for a protected premise should be immune to unauthorized tampering and compromising so that the security achieved by the system is not neutralized. For example, it should not be possible for a burglar or robber to defeat the security by breaking into the system and substituting a "bogus" transponder for a legitimate or valid one to avoid producing an alarm signal. Such a bogus transponder would provide a false indication to the controller that nothing is wrong and that conditions are normal, even though an unauthorized entry sensor, associated with the substituted transponder, may have been tripped. It is important for the controller to "know" if a valid transponder has been substituted with a similar transponder or any other device designed to respond like the substituted transponder. In addition, it is desirable that the security system function in such a way that an unauthorized person will not be able to tell, from the output of a transponder, whether an alarm has been triggered. When a "silent" alarm is employed, it is usually preferred that knowledge of that alarm actuation be withheld from the robber or burglar in order to allow time for law enforcement personnel to arrive.
The present invention achieves significantly greater protection against unauthorized tampering and compromising than that realized in the prior security systems. Furthermore this is accomplished at relatively little cost and requires very little space to implement. Among the very desired results obtained by the present invention, replacing of a transponder with a bogus one, or even with a computer, will not compromise the security and will be detected. Moreover, observation of the output of a transponder will not reveal whether an alarm has been actuated by that transponder. This is achieved in the present invention by encrypting or coding the data sent from each of the transponders, located within a protected premise or area, to the controller. In the past, for high security protection encryption has been employed in the communication link that leaves the controller, and its protected premise, and couples to a remote central station. Such prior systems, however, are not adaptable to the coding of the data from the individual transponders.